I thought it was vitamin C, but the “L” prefix had me ponder as to whether there was some dangling molecule making it incompatible with the formula. Not a chemist and there is no harm in asking. Thanks again!
Ascorbic acid is a chiral compound, which means it has two structural forms (enantiomers) that are mirror images of each other. The two forms are called L (for levo) and D (for dextro). The L enantiomer of ascorbic acid is the biologically active form and is commonly known as vitamin C. The D form has exactly the same chemical formula, but not the same 3D structure (it's the mirror image of the L form).
Since the L form is what's called for in the formula for FX-55, common ascorbic acid is indicated because it is the L form even though it's rarely written as L-ascorbic acid.
Chiral compounds are like gloves. There's a left-hand glove and a right-hand glove and you can't interchange them.
Chirality is important in biology. All amino acids that make up proteins are chiral (except for the simplest one, glycine) and all are of the L form. Naturally occurring sugars are all the D form. L-glucose can be synthesized, but when eaten it has no nutritional value.
It's slightly more complicated than that, but that's the basics.